Stranded tungsten wire incandescent lamp filament



April 28, 1970 L WALTER ET AL 3,509,411

STRANDED TUNGSTEN WIRE INCANDESCENT LAMP FILAMENT Filed Aug. 5, 1966 Fig.

Fig. 2.

In venfors John L. Walter Ken/rem 5. G Per/wee,

The/r Affomey United States Patent US. Cl. 313-343 4 Claims ABSTRACT OF THE DISCLOSURE A tungsten wire incandescent lamp filament is disclosed which has a much greater resistance to failure than conventional filaments, particularly under repetitive shock loads. The improved filament is manufactured by spirally twisting a plurality of round tungsten wires around a round central tungsten wire and drawing through a die to reduce the elfective transverse dimension of the composite to form a composite body having a circular cross-section with the central strand having a polygonal cross-section having a plurality of planar surfaces corresponding in number to the outer strands, the outer strands having corresponding planar surfaces in areal engagement with the planar surfaces of the central strand and planar surfaces in areal engagement with adjoining outer strands.

This invention relates to incandescent lamp filaments and more specifically to an improved multistrand tungsten filament having improved shock loading characteristics and to a lamp utilizing the improved filament.

It is well known that the history of the incandescent lamp, from its rudimentary beginnings and comparative inefiiciency to the present relatively eflicient device, closely involves the filament. Many substances, both metallic and nonmetallic, have been tried as filament materials in the continuing effort to further improve lamp efiiciency and life. Gradually most incandescent lamps, setting aside those intended for special use, came to have tungsten filaments, since a comparatively high ratio of light output to power input was obtained and mechanical properties were also better than those of most available filament substances. However, even tungsten was not without some shortcomings, particularly as the demands placed upon lamps by constantly expanding and advancing technologies increased. Lamps were called upon to do more than provide light under static conditions, they had to provide light reliably under severe operating conditions. For example, lamps used in automobiles and other ground transportation equipment must withstand shock loading conditions (vibration) while still delivering light for an acceptable service life. Even more stringent requirements must be fulfilled by lamps used in aircraft where vibration, particularly in some parts of the craft such as the tail and wing tips, can very drastically shorten the lamp service life. Needless to say, reliabilityin many of those situations is a premium requirement.

Since every year several billion electric lamps are produced throughout the world and the performance of these lamps depends mainly on the quality of the filament, there can be little speculation as to why improvement has been constantly sought in the tungsten filament. The brittle nature of tungsten results in manufacturing losses during coiling of the wire into the filament coil and the tendency for it to sag during lamp operation causes premature burnout. Sagging of the filament alters the distances between turns of the filament coil which in turn creates hot spots that ultimately result in coil burnout. Also, sagging alters the light emitting character of a lamp. Further, after a tungsten filament has been annealed, as occurs naturally as a result of lamp operation, it becomes even more brittle and susceptible to breakage. While the problems of sag and brittleness have been partially overcome by processing techniques such as using doped tungsten, they remain as problems significantly affecting lamp usefulness.

It was pointed out earlier that until the present time tungsten filaments for incandescent lamps were characteristically brittle in the annealed condition and have been subject to sagging, which reduces the service life. The problems arising from sagging have in the past been partially overcome by using tungsten doped with materials sucli as compounds containing aluminum, silicon and potassium. These doping materials cause the formation of a particular coarse, fibrous grain structure upon annealing heavily drawn wire containing them. Although doping alleviated the sagging problem somewhat, it nevertheless has remained a significant problem.

It is a principal object of this invention to provide a new filament wire for incandescent lamps which has improved ductility and shock resistance.

Another object of this invention is to provide a multistrand tungsten filament wire for incandescent lamps which has improved non-sag properties and increased strength and ductility under operating conditions where vibration exists during operation.

A further object of this invention is to provide an improved lamp utilizing the filament wire of this invention.

Other objects and advantages of this invention will be in part obvious and in part explained by reference to the accompanying specification and drawings.

FIG. 1 is a greatly enlarged, photomicrograph of a cross-section of a filament wire according to this invention; and

FIG. 2 is an enlarged side elevation of a filament wire of the type shown sectioned in FIG. 1.

Broadly, the filament wire of this invention is a multistrand tungsten filament in which a single center strand is surrounded by a plurality of additional strands which spiral around it. The cross-section of the central strand is polygonal, the exact number of sides depending upon the number of strands spiraled around it. The outer or surrounding strands each have an arcuate outer surface and substantially planar side and inner surfaces that result in areal contact between adjoining outer strands and between each of the outer strands and the single center strand. I

The present invention which provides increased filame nt ductility along with non-sagging characteristics has a unique multi-filament geometry previously unknown in the filament art. Specifically, referring to the FIG. 1 of the drawings, it can be seen that the filament wire 10 is comprised of seven individual strands and that there exists a single center strand 11 whose periphery is completely surrounded by the remaining outer strands 12, which are twisted around it as shown by FIG. 2. Further, it will be noted that between any two of adjoining outer strands 12 there exists planar contact as indicated at 15. Similarly, between any one of the strands 12 and the center strand 11 there exists planar contact as indicated at 16. In this case the center strand 11 is of a hexagonal cross-sectional shape since there are six surrounding strands 12 that must make intimate planar contact with it.

In the event that a greater number of outer strands 12 are used, the cross-sectional shape of center strand 11 sides equivalent to the total number of surrounding" strands. The particular geometry shown in the drawing is to be preferred since it delivers the highest packing fraction when initially coiling the strands 12 about the center strand 11.

The geometry provides, possibly by virtue of the planar interlocking relationship of the strands, a strong filament wire in which the flexure characteristic of a single wire is approached but which at the same time provides ductility and resistance to breakage that have not been previously attained. By way of comparison, a filament constructed only of individual strands of circular section would have only line contact between the individual strands (rather than planar contact) and mechanically would have the flexure qualities of a laminated beam rather than a unitary beam.

Further, it has been found that the geometry of the filament wire of this invention is one in which the lumen output of a lamp utilizing the filament wire is substantially constant throughout its life while with conven tional filaments a decrease in lumen output often occurs. Change in lumen output is especially true in a stranded filament using strands of circular cross-section since the geometry of the filament changes significantly during operation.

The filaments of this invention, as already stated, are produced by twisting together a plurality of tungsten strands into a multistrand filament in which a single center strand is surrounded by those remaining. The remaining or outer strands thus spiral around the single center strand. Following stranding or twisting together, the wire is drawn through dies to reduce it to final diameter and at the same time force the strands into intimate, planar contact.

The drawing operation causes the center strand to assume its polygonal cross-sectional configuration, the outer strands to have an outer arcuate surface and also causes the outer strands to develop planar side and inner surfaces which are in areal contact with the corresponding surfaces on the strand adjoining. Once the wire is drawn to final diameter, it can be twisted into coil form to form a filament and mounted on the usual type of supports within a lamp envelope for connection to a source of electricity.

Whereas the wire of this invention remains ductile following heating to above the recrystallization temperature of tungsten, a temperature which is exceeded during normal operation of an incandescent lamp, such is not the case with normal filaments which become more brittle after being subjected to a recrystallizing anneal or operated for a short period of time. For example, the stranded and drawn wire (about 12-mil diameter) can be readily bent about a radius as small as inch without fracturing.

To illustrate the effectiveness of the invention, several lamps were manufactured utilizing a stranded doped tungsten wire and subjected to life testing under shock loading conditions. Specifically, the filamentswere prepared by stranding seven mil diameter tungsten wires and drawing this wire down to final diameters of 4, 7 and 12 mils. The stranded and drawn wire was then fabricated into filaments by existing procedures and subjected to life tests under shock conditions. Whileburning,'the filaments were subjected to shock loads of about 400 gs of a frequency of 800 impacts per minute and further were subjected to an off-on cycle during the tests. Standard lamps were subjected to the same testing conditions so that a qualitative comparison could be obtained. The following table shows the service life of the standard lamps under the shock loading conditions.

. JWELEI Life in hours, Life in hours, Percent survivors standard lamps stranded lamp 61 34 90 43 96 51 101 72 106 78 Terminated A comparison of the service lives shown in the preceding Table I clearly show about a 100 percent increase in the life of stranded filaments over conventional filaments. However, a more telling fact is that inspection of the stranded filament lamps following failure showed that in not one instance had the stranded filament been responsible for interruption of operation. Thus, while the standard filament wire failed in fairly short times under the vibratory conditions, all of the stranded filaments were still operational. 1

Although the present invention has been described in connection with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What We claim as new and desire to secure by Letters Patent of the United States is:

1. An improved filament wire for incandescent lamps having improved resistance to brittle fracture and failure caused by sagging consisting of a plurality of tungsten strands twisted together into a multistrand filament in which a single center strand is surrounded by the remaining strands which spiral around it, the center strand having a substantially polygonal cross sectional shape and the surrounding strands eaclr having a substantially arcuate outer surface and substantially planar side and inner surfaces so that direct areal contact exists between the individual strands.

2. An improved incandescent filament as defined in claim 1 wherein seven strands are present and the cross sectional shape of the center strand is essentially hexagonal.

3. An incandescent lamp including an improved filament as defined in claim 1.

4. The filament wire as recited in claim 1 wherein said tungsten strands are completely recrystallized.

References Cited JOHN W. HUCK'ERT, Primary Examiner A. J. JAMES, Assistant Examiner US. Cl. X.R. 29- s05; 313 217, 344 

